Which harp for which key?
             IF BLUES IN             USE HARP IN
                  c                        f
             c sharp or d flat             f sharp
                  d                        g
                  e flat                   a flat
                  e                        a
                  f                        b flat
             f sharp or g flat             b
                  g                        c
                  a flat             c sharp or d flat
                  a                        d
                  b flat                   e flat
                  b                        e
N.B. Diatonics in all cases whilst it is possible to play blues on a chromatic it is very difficult due to the relative stiffness of the reeds.